Denitrification in groundwater: An undervalued ecosystem service

Shallow groundwater (<5 m deep) is often the predominant flow path for nitrate (NO₃⁻) leached from the root zone and transported to local surface waters. Denitrification occurring in the saturated zone reduces the load of NO₃⁻ that is discharged to groundwater-fed surface water bodies and by doing so offers an ecosystem service which is currently not accounted for

Denitrification in the shallow groundwater system of two agricultural catchments in the Waikato, New Zealand

Intensification and expansion of pastoral farming in New Zealand has resulted in increased nitrate (NO₃⁻) leaching. Nitrate leached from the root zone into the underlying groundwater may travel to surface waters and result in eutrophication. Denitrification in the groundwater system may help reduce the impact of intensification on the receiving waters by converting reactive NO₃⁻ to inert dinitrogen (N₂) gas. Little information exists about the occurrence of denitrification in groundwaters within New Zealand. This project investigates the extent of and limitations on denitrification in two contrasting small catchments within the Waikato region of New Zealand. The lowland Toenepi catchment is under high intensity dairying and features fine textured, alluvial and old volcanic ash deposits. The Waihora well field is an upland catchment under low intensity pastoral agriculture and is formed from coarse, young volcanic deposits. Isotopic analysis of NO₃⁻ in reduced groundwater samples from the Toenepi catchment showed temporal and spatial variation in the degree of δ¹⁵N and δ¹⁸O enrichments. Insufficient knowledge on groundwater flow paths at the study site as well as low NO₃⁻ concentrations made interpretation of the isotopic results difficult. Thus push-pull tests were performed but these, except one, were unable to demonstrate denitrification under in situ conditions, even when a carbon (C) substrate was added. However, laboratory incubations of aquifer material taken from the vicinity of the well screens demonstrated denitrification capacity was present. Addition of a C substrate (glucose) generally resulted in an increase in total N gas production. This response to C addition indicates that C availability limits denitrification in situ. However, even the low rates of denitrification measured could have a significant impact on the NO₃⁻ leaching into the shallow groundwater, provided flow paths, and therefore reaction time, are long enough. The isotopic analysis of NO₃⁻ in ground water samples proved ineffective at determining whether denitrification occurred at the Waihora field site, as most reduced groundwater samples had NO₃⁻ concentrations too low for analysis. However, laboratory incubations demonstrated that denitrification could occur below the root zone and was related to the presence of relict organic matter, in the form of buried soils and vegetation. Further experiments determined the denitrification potential of samples from below the A horizon, and indicated that much of the profile was C-limited as addition of glucose or hot-water extractable C resulted in an increase in total N gas fluxes. Despite the lower denitrification capacity at the Waihora well field (compared to Toenepi), the reduced portion of the profile has the potential to make a significant reduction in the NO₃⁻ concentrations in the shallow groundwater as N inputs from the land are much lower. This research has demonstrated that denitrification does occur in shallow groundwater systems in New Zealand, and that despite being limited by C-availability, significant reduction of NO₃⁻ leached from the root zone could occur as a result of relict organic matter located well below the soil zone in the groundwater system

Multi-pronged approach to elucidate nitrate attenuation in shallow groundwater

It is increasingly being recognised in New Zealand that denitrification occurring in the groundwater zone can result in a substantial reduction of the nitrate load leached from agricultural land before this load can reach water supply wells or discharge into groundwater-fed surface water bodies. This natural attenuation process provides an ecosystem service with regard to the protection of the quality of our freshwater resources that to date has not been adequately accounted for. This is largely due to the major challenges involved in trying to understand and quantify the denitrification occurring in a particular water management zone. Based on the example of research conducted at the 'Waihora' site in the Lake Taupo catchment, we demonstrate a multi-pronged approach to elucidate the biogeochemical and hydrological controls on denitrification. The site is unique in New Zealand inasmuch as it has allowed investigating shallow groundwater underlying a pastoral hillslope in great detail using 11 multi-depth well clusters (comprising 26 wells in total). As denitrification is only active under mildly reduced conditions, a systematic approach to characterise redox conditions based on measured concentrations of dissolved oxygen, nitrate, dissolved manganese, dissolved iron, and sulphate provided fundamental initial information on the denitrification potential of the groundwater system. Determining stable isotope signatures of nitrate (δ¹⁵N, δ¹⁸O) and excess N₂ dissolved in the groundwater can help differentiate between denitrification potential and denitrification that has actually occurred in a given groundwater sample. As the interpretation of these data is strongly dependent on the understanding of the temporal and spatial variation of groundwater flows at the site, hydrological understanding proved critical. Tritium, chlorofluorocarbons, and silica were determined on selected groundwater samples to gain insight into the distribution of groundwater mean residence times ('ages') at the field site and slug tests provided estimates of the hydraulic conductivity of the different deposits found in the shallow groundwater system. Given that most biogeochemical and hydrological parameters analysed showed substantial spatial variation, hydrological modelling of the hillslope proved the only promising way to ascertain the overall effect denitrification may have on the groundwater nitrate discharges from this site

Investigating how soil drainage class affects the redox status of shallow groundwater

We have recently extended our research on the assimilative capacity of groundwater systems for nitrate to the Reporoa Basin in Waikato. The particular focus of the presented study is to investigate to which extent the drainage class of the soil zone (approx. 1 m depth) affects the redox status of the underlying shallow groundwater. The Reporoa Basin offers good opportunities to address this question as soils ranging in their drainage class from well-drained to very poorly drained occur in close proximity and at locations with reasonably shallow depth to the groundwater table

Groundwater assimilative capacity: Evidence for and quantification of denitrification

It has become evident in recent years that many groundwater systems exhibit some degree of assimilative capacity for nitrate. Denitrification is the key component of this assimilative capacity, as it is the only process that permanently removes reactive nitrate from the groundwater by converting it under oxygen-depleted conditions into gaseous N (predominantly N₂). To be able to ‘Manage within Limits’, as stipulated in developing land and water resource policy, we need to ascertain the maximum nitrate leaching losses that can be accepted in a catchment without violation of the water quality standard agreed for a given monitoring site. Being able to quantify any denitrification that may occur along the groundwater flow paths to the monitoring site is crucial for the establishment of such catchment-scale cause/effect relationships, as denitrification can represent a significant buffer in these relationships

Fate of a dairy cow urine pulse in a layered volcanic vadose zone

Nitrate-N leaching from dairy cow urine patches has been identified as one of the major contributors to groundwater contamination and degradation of surface waters in dairying catchments. To investigate the transport and transformations of nitrogen (N) originating from urine, fresh dairy cow urine was collected, amended with the conservative tracer chloride (Cl) and applied onto a loamy sand topsoil, underlain by gritty coarse sands and pumice fragments in the lower part of the vadose zone. The fluxes of the different N components and the conservative tracer leaching from the urine application were measured at five different depths in the vadose zone using three Automated Equilibrium Tension Lysimeters (AETLs) at each depth (max. 5.1 m). The uppermost part of the saturated zone was also monitored for the leached N and Cl fractions from the urine application. Textural changes and hydrophobicity in the vadose zone materials resulted in heterogeneous flow patterns and a high variability in the N and Cl masses captured. All three forms of potentially leachable N from the urine – organic-N (org-N), ammonium-N (NH4-N) and nitrate-N – were measured at the bottom of the root zone at 0.4 m depth. At the 1.0 m depth, effectively all of the captured N was in the mobile nitrate-N form. In the lower part of the vadose zone at 4.2 m, 33% of the applied urine-N was recovered as nitrate-N. This fraction was not significantly different from the corresponding fraction measured at the bottom of the root zone, indicating that no substantial assimilation of the nitrate-N being leached from the root zone was occurring in this vadose zone

Denitrification capacity and potential in the Toenepi catchment

A discrepancy is often observed between the amount of nitrate (NO3-) estimated to be leaching from the root zone of agricultural land, and NO3- concentrations measured in associated surface waters. This reduction in NO3- concentrations is often the result of denitrification in the shallow groundwater. Determining where, when and how much denitrification occurs in situ can be challenging, especially when flow paths in a catchment are not well defined. Laboratory incubation experiments can be used to investigate denitrification and understand the factors limiting the process in situ

Understanding the denitrification capacity and potential of a volcanic plateau site

The assimilative capacity of a groundwater system largely depends on the aquifer matrix. Volcanic profiles may have the potential to assimilate significant quantities of nitrate (NO₃⁻) due to the presence of relict organic matter in the form of palaeosols (buried soils) and vegetation covered by deposits during the eruption. This organic material could provide the necessary electrons required to fuel heterotrophic denitrification, and reduce environmentally damaging NO₃⁻ to inert dinitrogen (N₂) gas

Complex groundwater flow paths in a small Taupo hillslope

The Waihora research site is a small (0.7 ha) pastoral hillslope that drains into the headwaters of the Tutaeuaua Stream on the northern shores of Lake Taupo. To understand how the strong vertical redox stratification that has been observed at several well sites (Stenger, 2011) affect the groundwater system’s assimilative capacity for nitrate it is essential to elucidate the 3D groundwater flow paths. Early modelling studies assumed a sloping water table with linear flow following the land surface topography downhill towards the wetland/stream (Woodward et al., 2009). However, recent tracer studies in several wells have found the principal direction of tracer movement to be quite unexpected, with diagonal transport across the hillslope